The reproductive ecology of resident manta rays (Manta alfredi) off Maui, Hawaii, with an emphasis on body size

In resident manta rays (Manta alfredi) off Maui, sexual maturity appears delayed until growth exceeds 90% of maximum size, an indicator that large body size provides a reproductive advantage at the expense of a shorter reproductive time period. In this study, 286 surveys were conducted between 2005 and 2010 using photo-identification and photogrammetry to study the reproductive ecology of a resident population of manta rays off Maui, Hawaii, and investigate the reproductive benefits of large body size in each sex. Although reproductive activities occurred year-round, mating trains and late-term pregnant females were significantly more likely to be observed during the winter months. Some females were pursued by males during both winter and summer of the same year, suggesting multiple ovulations may be possible in a single year. Males likely detect a female's reproductive state by positioning directly behind her, or passing through her bodily excretions. The mean pregnancy rate was estimated at 0.56 pregnancies/adult female/year with larger females pregnant more often, and more likely in consecutive years. The operational sex ratio was heavily skewed with 2.68 adult males per reproductively available female. Although males appear to compete with one another for females within a mating train, no direct physical competition was ever observed between males. Evidence of highly dynamic mating trains lasting more than one day suggests endurance rivalry may be the primary mating strategy among males, during which larger males may benefit from greater energy reserves. The study area appears to be an important staging area for mating individuals in this population.